(a) Field of the Invention
The present invention to an objective lens system for microscopes, designed for magnification level on the order of 20.times., and more specifically to an objective lens system so designed as to be usable for vertical illumination fluorescence microscopy.
(b) Description of the Prior Art
Conventionally, there has been no objective lens systems designed especially for fluorescence microscopy, and ordinary type of objective lens systems for microscopes showing relatively high transmittance for excitation light (mainly ultraviolet rays) were selected for fluorescence microscopy. However, ordinary type of objective lens systems designed for magnification levels on the order of 20.times. have a numerical aperture (NA) value of 0.45, and even when they have relatively high transmittance, it was not high enough for fluorescence microscopy. In such objective lens systems, excitation light is attenuated rather remarkably before reaching a specimen as compared with intensity of incident light. Further, since fluorescence emitted from a specimen is very weak, objective lens systems having a small NA value remarkably darken images observed fluorescence microscopes. In order to eliminate these defects, it is required to design an objective lens system which shows high transmittance for excitation light and has a large NA at the same time. Moreover, an objective lens system having a large NA value is preferable for enhancing resolution also for purposes other than fluorescence microscopy.